Syria's government envoy at the peace talks in Astana on Monday denounced as "provocative" and "insolent" a speech delivered by the head of the rebel factions attending the gathering in Kazakhstan.

Bashar Ja'afari, Syria's UN ambassador, said rebel leader Mohammad Alloush's speech in Kazakhstan’s capital was not appropriate for a gathering of diplomats.

Ja'afari in remarks to reporters in Astana repeatedly referred to the rebel delegation as representatives of "terrorist armed groups". He also said that the agenda for the talks sponsored by Russia, Turkey and Iran is "not ready yet."

The harsh and uncompromising tone of Ja'afari's remarks is a bad omen for the talks, which had barely started before rebel leaders refused to negotiate face-to-face with representatives of the Syrian regime.

The talks had been billed as the first time armed rebel groups were due to negotiate with President Bashar al-Assad's regime since the conflict erupted in 2011.

But rebel spokesman Yehya al-Aridi told AFP the opposition backed out of the first round of direct talks because of the regime's continued bombardment and attacks on a flashpoint area near Damascus.

"The first negotiation session will not be face-to-face because the government hasn't committed until now to what it signed in the December 30 agreement," Aridi said, referring to the fragile ceasefire deal brokered by Turkey and Russia.

Will the two-sides negotiate?

It remained unclear whether the two sides would negotiate directly later.

“It doesn’t mean that the two won’t meet,” FRANCE 24 correspondent Thomas Loewe said. “But it does mean that these last few weeks of preparation haven’t come to the fruition that was hoped for.”

Several rounds of failed talks in Geneva saw political opposition figures take the lead in negotiating with the Damascus regime.

But in Astana, the 14-member opposition delegation is composed solely of rebels leading the armed uprising, with members of the political opposition serving as advisors.

The two delegations entered a luxurious meeting room for opening statements by the Kazakh foreign minister, before the closed-door talks began.

“The rebel groups are in one room … and in the other are representatives of Bashar al-Assad and the Syrian government, and the mediator is going between the two groups,” FRANCE 24's Loewe explained.

The negotiators have been welcomed by all parties in the war, but the two sides arrived with apparently divergent ideas on their aim.

Rebel groups say the talks will focus on bolstering the ceasefire, but Assad has insisted rebels lay down their arms in exchange for an amnesty deal.

Damascus has also called for a "comprehensive" political solution to a conflict that has killed more than 310,000 and displaced more than half of Syria's population.

"The government delegation took part in the Astana meeting on the basis that the agenda would include reinforcing the ceasefire and discussing the principles of a political solution," a source close to the government delegation told AFP.

The source said the Turks, Russians, and Iranians -- joint organisers of the talks -- were rushing to put together a final statement that the rebels and regime were expected to sign Tuesday.

'Common ground'

The talks come a month after the regime recaptured rebel areas of Aleppo, scoring its biggest victory since the war began.

"This is not a replacement for the Geneva process," rebel negotiator Fares Buyush told AFP, referring to the UN-hosted political negotiations set to resume in the Swiss city next month.

Delegation spokesman Osama Abu Zeid said the rebels were concerned with "more than just a ceasefire".

"The issue is putting monitoring, investigation, and accountability mechanisms in place," he told AFP. "We want these mechanisms so that this doesn't play out over and over."

Previous pushes for a long-term ceasefire have faltered, with both sides trading accusations over violations.

Syrian state media reported the regime had met the Iranian delegation as well as UN Syria envoy Staffan de Mistura ahead of Monday's talks to discuss their positions.

The regime will also seek to "consolidate the cessation of hostilities" and separate the rebels from the Islamic State group and former Al-Qaeda affiliate Fateh al-Sham Front, he said.

As the sides headed to Astana, three regime air strikes killed nine civilians in rebel-held areas in the central Syrian province of Homs, a monitoring group said.

'No shortcut'

Although Russia and Turkey have backed opposing sides in Syria, they have worked hand-in-hand in recent weeks to try to secure an end to the brutal war.

The Astana talks will be a major test of this new partnership.

The rapprochement, which saw Russia and Turkey conduct their first joint air strikes against IS targets in Syria last week, has come to fill the vacuum left by Washington's disengagement from the conflict in recent months.

US President Donald Trump's administration was invited to participate in the talks but did not send a delegation.

Washington will instead be represented by its ambassador to Kazakhstan, the State Department said, while a European diplomatic source said France and Britain would also be represented at the ambassador level.

Experts say a breakthrough could see some of the armed opposition join next month's Geneva talks.

"Nearly six years of war demonstrates there is no shortcut to ending it," a Western diplomat told AFP.

"A genuine transition in Syria first means building confidence on the ground. That is what the opposition have demanded and it's not so much to ask."